1. Field of the Invention
The present invention relates to techniques for driving high-side and low-side switching devices. More specifically, the invention relates to methods and circuits that drive high-side and low-side switching devices with alternating high-side and low-side pulses of varying duration or width to provide soft start and dead time between switching.
2. Description of the Related Art
Various driver circuits with high-side and low-side outputs are commercially available. A typical half-bridge driver, for example, provides alternating high-side and low-side output pulses to the gates of high-side and low-side power transistors. Examples include the IR2152 and IR2153 self-oscillating half-bridge driver integrated circuit (IC) products sold by International Rectifier Corporation, some features of which are described in U.S. Pat. No. 5,545,955, the disclosure of which is incorporated herein by reference in its entirety.
In the IR2152 and IR2153 products, the driver is packaged in a conventional DIP or SOIC package. The package contains internal level shifting circuitry, under voltage lockout circuitry, dead time delay circuitry, and additional logic circuitry and inputs so that the driver can self-oscillate at a frequency determined by external resistors and capacitors. These and other driver circuits provide dead time between high and low output pulses to prevent cross conduction, which occurs if both transistors conduct at the same time.
U.S. Pat. No. 6,002,213 discloses a MOS gate driver (MGD) circuit with high and low side dead time delay circuits that provide time delay intervals. The MGD circuit also includes a dead band control circuit that receives a feedback signal from a load circuit and, in response, controls the duration of the time delay interval.
In some applications, it is important to modify driver output pulses at start up. The IR2157 and IR21571 products sold by International Rectifier Corporation provide fully integrated ballast control ICs for fluorescent lamps. The IR2157 and IR21571 products include drivers and feature a start-up procedure that insures a flash-free start without an initial high voltage pulse across the lamp as well as various other features relating to lamp operation. Similarly, U.S. Pat. No. 5,932,974 discloses a lamp ballast circuit with MOS-gated power transistors connected in a half bridge to drive a gas discharge lamp. A self-oscillating driver circuit drives the transistors, and a soft-starting circuit gradually increases voltage across the lamp prior to ignition. The circuit also provides built-in dead time.
It would be advantageous to provide a gate driver circuit with improved soft start and dead time techniques.
The present invention provides a circuit for driving alternately driving high-side and low-side switching devices of, for example, a half-bridge, a full-bridge, or a push-pull primary. The circuit of the present invention produces drive pulses of varying durations (also referred to herein as pulse xe2x80x9cwidthsxe2x80x9d) to control operation of the switching devices, in particular relating to the implementation of soft start and dead time.
The high-side and a low-side pulses produced by the circuit of the present invention are of approximately equal duration, thus providing balance. The high-side and low-side pulses are preferably separated by dead time.
To provide soft start, the pulses have a duration or pulse width which increases from zero to a maximum. The pulses are separated by dead time, which can decrease as the pulses increase to the maximum duration. The dead time has a minimum duration when the pulses have a maximum duration. As a result, soft start can be provided without complex circuitry for modifying voltage or oscillator frequency.
The pulse circuitry is preferably implemented with an oscillator that provides a sawtooth-like signal; each period of the oscillator signal includes a rising portion followed by a falling edge to a low portion. The pulse circuitry includes reference circuitry that provides a varying reference signal. The pulse circuitry also includes a comparator that responds to the periodic signal and to the varying reference signal by providing a pulse output signal whose pulse width is proportional to the reference signal and which terminates at the falling edge of the oscillator signal.
In this implementation, the oscillator also provides a gate pulse after the falling edge of alternate periods of the oscillator signal, so that the period of the gate pulses is half that of the first oscillator signal. The reference circuitry includes circuitry providing a rising signal that increases from start up to a maximum value. The reference circuitry can also include sampling circuitry that responds to each gate pulse by sampling the value of the rising signal; the reference circuitry can obtain the varying reference signal from each sampled value until the next gate pulse. As a result, two pulses provided between consecutive gate pulses, one each for the high-side and low-side switching devices, are approximately equal in duration.
The present invention is preferably provided in the form of an integrated circuit for driving high-side and low-side switching devices. The integrated circuit includes high-side and low-side output pins for connecting respectively to the gates of the high-side and low-side switching devices. The integrated circuit also includes pulse circuitry that provides pulses alternately through the high-side output pin to turn on the high-side switching device and through the low-side output pin to turn on the low-side switching device. The pulses vary in duration to set dead time and soft start.
The pulse circuitry includes oscillator circuitry, reference circuitry, and a comparator as described above. Further, the oscillator circuitry includes oscillator resistance and capacitance pins for connecting to an external resistance and an external capacitance to determine the frequency of the periodic signal. The reference circuitry includes increasing circuitry and sampling circuitry as described above, and the increasing circuitry includes a reference voltage pin for connecting to an external capacitance that increases in voltage as the capacitor initially charges. In this way, the reference voltage pin provides the rising signal.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.